In order to provide synchronous communication in a communication system (such as a Global System for Mobile (GSM) Communications, a Third Generation of Mobile Telecommunications Technology (3G) system, a Long-Term Evolution (LTE) system, etc), the system usually employs a mechanism for providing a transmitter and receiver with a common communications time reference, up to a certain precision.
A time reference (which may also be referred to as a timing reference) is a reference for use by devices in a communications network in timing their communications with other devices. For example, synchronous systems based on Orthogonal Frequency Division Multiplexing (OFDM) signals such as LTE may be designed under the assumption that the transmitter and receiver have time references whose difference does not exceed the duration TCP of the cyclic prefix. The objective may be that the signals arrive at the receiver (for example, a user equipment (UE) device or an evolved NodeB (eNB)) within the interval (Tref, Tref+TCP), where Tref is the time reference at the receiver. This allows the receiver to correctly place its sampling window and reduce inter-carrier interference.
In cellular communications, the mechanism employed by the system may exploit the fact that a base station (for example, an eNB in an LTE system) is present at one end of a cellular communications link. For example, in downlink (DL) transmission (for example, from a serving eNB simultaneously to one or more UE devices), downlink timing is commonly used by each device served by a base station to derive a time reference based on signalling transmitted by the serving base station. In this case, it is possible to obtain a satisfactory time alignment because each device only needs to synchronize to a single transmitter (i.e. the transmitter of the serving base station).
In uplink (UL) transmission (for example, simultaneously from one or more UE devices to the serving eNB), uplink timing is commonly used by the serving base station to derive the time reference for each device served by the base station. The serving base station then communicates the derived time reference using appropriate signalling. In this case, it is possible to obtain a satisfactory time alignment because the transmitters of each device need to align with the same receiver (i.e. the receiver of the serving base station).
However, both downlink and uplink timing will depend on the topology of the network (for example, the distance between transmitters and receivers, etc). Moreover, the time reference that is derived is only valid for communications to or from the base station from which it is derived. This means that, if a device needs to communicate with a different base station it will first need to acquire the appropriate time reference.
This problem is particularly apparent in the case of device to device (D2D) communications within the communications network, since there may be several transmitters communicating with several receivers. In particular, in this case, each transmitter needs to have a time reference with each receiver within the intended communication range. However, establishing such common time references is complicated by the fact that the topology of a device to device network may change quickly.
For example, the targeted coverage area around a device to device transmitter may cut across the boundary of one or more cells of the communications network. This means that, in some cases, the uplink and downlink timing schemes mentioned above may not result in common or compatible time references. This may, for example, depend on the geometry of the cells and the positions of the device to device nodes. For example, if the device to device transmitter and the device to device receiver are located on different (for example, neighbouring) cells, the uplink and downlink timing schemes may not result in common or compatible time references.
Similarly, if the device to device transmitter is located close to its serving base station whereas the intended device to device receiver is located far away from its serving base station, the resulting time references at the devices may be incompatible for direct device to device communication. Existing methods for deriving common time references would produce timing discontinuities between devices. The problem is exacerbated by the fact that the base stations in the cells would not normally share a common time reference. In addition, it would not be possible for devices that are out of coverage to derive a valid time reference based on uplink and downlink timing schemes that require signalling by the network.
Therefore, there is a need for an improved method for deriving references (such as time and/or frequency references) to be used by devices for communications in a communications network, which does not depend on the topology of the network to thereby allow improved synchronization.